Hyperfine interactions and local environment effects in FexNi100−x (x = 55-67) Invar alloys: 57Fe Mössbauer spectroscopy data at 5 K

A systematic study of the magnetic hyperfine field distribution for 57Fe in the Invar alloys FexNi100−x (x = 55, 60, 63, 65 and 67) have been performed by Mössbauer spectroscopy technique at 5 K. The composition dependences of the magnetic hyperfine fields (Bhf), isomer shifts and relative intensities of the magnetic subspectra were measured and analyzed. The reliable data on the correspondence between the Bhf magnitude and the type of locale atomic configuration were obtained. It is confirmed that the Fe-Ni Invar alloys in their ground state are predominantly collinear ferromagnets with well-defined atomic magnetic moments. Particular emphasis has been placed on the low-field (LF) component of the distribution (Bhf = 1.4 and 2.5 T) which is considered as corresponding to the Fe sites with the antiferromagnetic (AFM) alignment of the magnetic moment. The most striking feature of the LF component is the anomalous positive isomer shift that corresponds to a large reduction of the local electron density at the Fe sites. It may be proposed that the volume effect is one of the plausible reasons for the increase in the IS value for LF components. In this case, we should suggest that the formation of the AFM sites leads to some increase in the local atomic volume. The possible influence of the competition between energetically satisfied and unsatisfied exchange bonds on the stability of the AFM states of the Fe atoms is briefly discussed.